Presently, more and more Liquid Crystal Display device (LCD TV for example) adopts LED backlight, which has advantages of long service life, saving power, and easy to drive.
Referring to FIG. 1, it schematically shows a voltage boost driving circuit for LED backlight of an embodiment of prior art. The boost driving circuit includes a first voltage boost unit 10, a second boost unit 20, an electronic switch K, an isolating diode D3, and a control circuit 30. The first voltage boost unit 10 includes a first storage inductor L2, a first diode D2, and a first capacitor C2 connected to the two ends of the input power source U2 in serial. The second voltage boost unit 20 includes a second storage inductor L3, a second diodes D4, and a second capacitor C3 connected in serial, where the second voltage boost unit 20 is connected to the first capacitor C2 in parallel. An end of the electronic switch K is connected to the anode of the second diode D4, another end of the electronic switch K is connected to the negative pole of the input power source U2. The anode of the isolating diode D3 is connected to the anode of the first diodes D2, the cathode of the isolating diode D3 is connected to the anode of the second diode D4. The control circuit 30 includes a PWM control integrated circuit IC2 and a resistor serial connected in parallel with the second capacitor C3, where the resistor serial includes a first resistor R3 and a second resistor R4. The output port of control signal of the PWM control integrated circuit IC2 is connected to the cathode of the isolating diode D3 via the electronic switch K, while the feedback signal input port thereof is connected to the common end of the first resistor R3 and the second resistor R4.
The operating principle of the voltage boost driving circuit for LED backlight is that: when the electronic switch K is turned on, the first voltage boost unit 10 starts to work, and the input power source U2 charges the first storage inductor L2. When the electronic switch K is turned off, the second voltage boost unit 20 starts to work, the first capacitor C2 discharges, and the second storage inductor L3 starts to storage energy so that the voltage added on the second capacitor C3 increases, which further leads a result that the output voltage VO2 meets the voltage requirement of the LED backlight. Specifically, when the electronic switch K is turned off, the output port of control signal of the PWM control integrated circuit IC2 is connected to the cathode of the isolating diodes D3. When the electronic switch K is turned on, the connection between the output port of control signal of the PWM control integrated circuit IC2 and the cathode of the isolating diodes D3 is disconnected. In the present embodiment of the voltage boost driving circuit for LED backlight, the voltage boost value can be adjusted via controlling the duty ratio of PWM signal from the PWM control integrated circuit IC2 of the control circuit 30. When the electronic switch K is turned on, the first storage inductor L2, the isolating diode D3, and the electronic switch K constitute a loop. The voltage between the two ends of the first storage inductor L2 equals to the voltage of the input power source U2, so that the input power source U2 charges the first storage inductor L2. In the meantime, the voltage between the two ends of the second storage inductor L3 equals to the voltage UC2 of the first capacitor C2, the voltage UC2 therefore charges the second storage inductor L3. When the electronic switch K is turned off, the overlapped voltage of the energy stored in the first storage inductor L2 and the input power source U2 discharges to the first capacitor C2, so that the voltage UC2 of the first capacitor C2 is increased. At the same time, the overlapped voltage of the energy stored in the second storage inductor L3 and the boosted voltage UC2 discharges to the second capacitor C3, so that the output voltage UC3 of the second capacitor C3 multiples the voltage of the input power source U2. During the boosting process, when the electronic switch K performs the operation of on and off, the voltage boost results of the first voltage boost unit 10 and the second voltage boost unit 20 are independent from each other due to the existence of the isolating diodes D3. After the boost step described above, if the first voltage boost unit 10 increases the input voltage of the input power source U2 by 5 times, the second voltage boost unit 20 also increases the output voltage of the first voltage boost unit 10 by 5 times, which means the voltage boost driving circuit for LED backlight increases the input voltage of the input power source U2 by 25 times. Regarding each boost time of the first voltage boost unit 10 and the second voltage boost unit 20, those who skilled in the art would how to adjust it by adjusting the divider resistance or other means having the same function.
Although the LED light bar of voltage boost driving circuit for LED backlight shown in FIG. 1 can fulfill the driving requirement of LED light bar of high voltage, for LED light bar of side-style, when the voltage added to the LED light bar is too high, the voltage of the LED light bar relative to the ground is also high, which results in potential risk.